Nicholas Prosper, 19, has been sentenced to a minimum of 49 years in prison for murdering his mother, brother and sister in Luton last year
The bodies of Juliana Falcon, 48, Kyle Prosper, 16, and Giselle Prosper, 13, were found at their home in September
After killing his family, Prosper planned to carry out a mass shooting at his former primary school - having chosen a Friday 13th date deliberately
Mrs Justice Cheema-Grubb says Prosper "intended to unleash disaster on the community of Luton" and was "inappropriately cheerful" after being arrested
The court heard Prosper hoped to kill his family while they slept - but after a noisy struggle, he left the flat and neighbours called police
Watch Prosper being arrested here and him buying his shotgun in a car park here
Watch: Teenager sentenced to minimum of 49 years in prison
Mrs Justice Cheema-Grubb says she will not be passing sentence today.
Instead, the judge says she needs time to consider all the factors put before her.
She tells the court her sentencing remarks will begin at 10:30 GMT on Wednesday.
CCTV footage the day before the murders shows Nicholas Prosper buying a gun from a man in a car park after showing him a fake firearms certificate.
He is then seen returning to his block of flats and entering a lift minutes later, carrying a bag.
Concluding his mitigation, Bentley says the case before the court is "incomprehensible".
"Something went very wrong with this young man in the year or so before these offences were committed," the barrister says.
"What I would invite the court to consider here is a young man at the start of his adult life.
"He has a diagnosis of autistic disorder. That is something that does at least provide context for how he got into the narrowing world he did on the internet.
"We would invite the court not to pull the rug at this early stage and say that is it."
With that, Bentley takes his seat.
Brian Farmer
Reporting from Luton Crown Court
Prosper has become increasingly fidgety in the past few minutes.
He's gone from examining his finger nails, to resting his head on his fist and then his hand.
Everyone in court is listening intently to David Bentley KC making arguments on his behalf.
Bentley tells the court Prosper had "retreated into a harmful, internet world" and was "isolated from the real world".
He says the court must give the killer credit for pleading guilty "at the earliest possible opportunity".
"From the outset he would accept responsibility for causing the deaths," Bentley argues.
The court was shown a video Prosper filmed ahead of the murders.
Brian Farmer
Reporting from Luton Crown Court
Prosper is bowing his head and scratching his hair while his barrister argues on his behalf.
The court is told his parents separated when he was nine years old and the relationship he has with his father is "strained".
In mitigation, Bentley says Prosper was living with an undiagnosed neurological development disorder in ASD.
He points to the defendant's lack of any previous convictions and his young age and stressed this must be taken into account.
Bentley says for "whatever the reason", the school shooting plan had not been carried out.
He also claims the forensic psychiatrist "failed to take into account" personality disorders when it came to his report.
"He is a young man who has gone down an internet wormhole," Bentley adds.
Image source, Bedfordshire PoliceBedfordshire Police have just released an image of Prosper inside the kitchen at the family home.
He is wearing the "distinctive" yellow and black outfit he thought people would remember him by after carrying out his planned school shooting.
Prosper is also shown using a plank of wood, pretending to aim it, as if it was a gun.
With that, Cray concludes the prosecution's evidence and sits down.
David Bentley KC is now on his feet to deliver mitigation on Prosper's behalf.
He starts by saying this is a "hugely distressing case" and nothing he says is an attempt to water down the tragedy of what has happened.
We've just heard a statement read on behalf of Ray Prosper, the former husband of Juliana Falcon and father to Nicholas, Giselle and Kyle.
"The pain of our loss will never be healed, this includes my whole family. Our lives will never be the same," he says.
"When I heard the horrendous news that day, part of my soul died too. This is a lose-lose situation for us all and we have lost four family members."
Mr Prosper ends by thanking the police and the community for their support following the murders.
He says the "awful incident" has affected many people and asks for privacy as the family attempts to move on.
In his conclusion, the psychiatrist stressed Prosper's offending was borne out of a "lack of empathy for others" and his desire to become famous.
He says there was no impairment in his rational judgement at the time of the murders.
Referencing Prosper's autism, the psychiatrist added: "The defendant's condition did not reduce his culpability.
"These offences were highly and meticulously planned. He understood executing his plan would cause a significant amount of harm."
The prosecutor resumes by telling the judge about a report on Prosper’s mental health, which was written last month.
Cray says this report, which contains interviews with Prosper’s father, Ray, was authored by a forensic psychiatrist.
It says no concerns were raised about his mental health when he was at school. He was described as a "quiet, geeky boy with a small group of friends who were into computers".
However, he became problematic when starting his A-level studies, Cray explains.
His teachers believed he had Autism Spectrum Disorder (ASD), but Prosper “refused” to talk to any mental health services and thus no diagnosis was made.
The forensic psychiatrist concluded Prosper was autistic and had "psychopathic traits".
One of the overarching questions from this court case has been around how Nicholas Prosper managed to buy the shotgun he killed his family with.
The court heard he included his own name, a photograph and his real address on a fake firearms licence he created.
The defendant also inserted the signature of a Bedfordshire Police firearms sergeant on to the forged document on 30 August.
On the same day, Prosper messaged a private seller who had advertised a shotgun on gunstar.co.uk for £450, and offered to pay him £600 if cartridges were included
The seller then agreed to drop the gun off to Prosper on 12 September, prompting the defendant to say in a message: "I look forward to meeting you."
Police said a full investigation was carried out into the gun seller and no criminal offences were identified.
Image source, Julia Quenzler/BBCProsper has, at times, held his head in his hands while the evidence has been read to the court
Court 5 at Luton Crown Court has been reopened and all parties are filing back into the room.
Judge Mrs Justice Cheema-Grubb takes her seat and Nicholas Prosper returns to the dock.
Timothy Cray KC, who is prosecuting the case, is back on his feet and about to continue reading the evidence.
Image source, Brian Farmer/BBCAssistant Chief Constable John Murphy spoke at a press conference after Prosper pleaded guilty to the murders
Reacting to Prosper's guilty pleas last month, Bedfordshire Police's assistant chief constable, John Murphy, said: "We understand the deep shock this case will have on people within Luton and across the country.
"Juliana, Kyle and Giselle were brutally murdered in their home by someone intent on causing the maximum harm and distress on not only his family, but to the wider community.
"We are extremely grateful to our officers who stopped him and prevented him from going through with his plans to target a school."
For any readers just joining us, here is a recap:
Shortly before the lunch break, the court was told of Prosper's planning for a mass shooting which he set out in documents that were seized whilst he was remanded in prison.
In those, Prosper insisted his plan "wasn't stupid" and that he wanted to have time to "cannabilise my family and rape a woman at knifepoint" before the shooting.
These documents then outlined a list of what he wanted to do. He wrote:
He concluded: "ONE OF THE BIGGEST EVENTS EVER"
After a morning of harrowing evidence, Mrs Justice Cheema-Grubb has adjourned proceedings for lunch.
We will continue bringing you the latest from court this afternoon.
A reminder there is support available for anyone affected on the BBC Action Line.
Nicola Haseler
BBC News
Prosper had his head in his hands as his extensive planning and ideology was exposed to the court.
He seemed particularly dismayed about the prosecutor saying he wanted to be famous.
